Concurrent with the development of the technological capability of putting satellites into an earth orbit, there has also developed a technology of diverse purposes for which these satellites can be used.
In many of these known uses the level of system performance could be greatly enhanced, and some new uses would becomes feasible, if a highly efficient, highly directional communications link between the earth and the orbitting satellite could be developed. For example, a highly directional communications link would tremendously increase the difficulty to an enemy of either jamming or "listening in" on a secure message system. There is, of course, the further advantage that a highly efficient, highly directional communications link that does not require a large amount of power for the electromagnetic beam steering, will lower the satellite electric power load, an improvement which is very desirable for obvious reasons.
Prior to the present invention, communications links between the orbitting satellites and the earth have been relatively inefficient, broad beamed and usually very frequency limited. The reasons for this include the use of spin stabilization to spatially orient the satellite (considered to be a "must" in almost all systems). This satellite spin has effectively inhibited the use of highly directional antennas, such as phased array antennas, although unsuccessful efforts have been made to mechanically despin the array antenna structure, i.e. the antenna array and the main satellite body spin in equal and opposite directions. Problems associated with coupling the electromagnetic energy from the main satellite body to the antenna array have been found to be so insurmountable as to preclude any serious implementation of the despinning antenna array concept. It is to be noted, however, that mechanically despun parabolic antennas featuring narrow electromagnetic beams have been used. These antenna systems are not entirely satisfactory because they are very slow in steering the electromagnetic beam and do not permit simultaneous transmission of different beams in different directions and have relatively high power requirements for the beam steering.
Other prior communications links have used travelling wave tube (TWT) amplifiers to energize antenna systems which do not attempt beam steering but radiate a cone shaped electromagnetic beam that waste most of the radiated energy in illuminating vast areas of the earth for which there is no need for coverage. The indiscriminate broadcasts can also create security, i.e. "listening in" problems. Further, in order to achieve even a reasonable efficiency, such as 20%, with the TWT, the bandwidth of prior satellite-earth communications links have had to be considerably reduced, accompanied by a reduction in the number of communication channels that can be handled.